It is known that low-voltage electrical systems, particularly for industrial-type applications, characterized by operating voltages lower than 1000 volts and by electric currents of relatively high nominal value producing considerable power levels, generally use current interruption and protection devices, known as automatic power circuit breakers.
Depending on the applications, these devices comprise one or more electric poles that constitute the interruption section of the circuit breaker; each pole comprises at least one arc chute and two electrical contacts, a fixed one and a moving one, which can be mutually coupled/uncoupled; in turn, the contacts are connected electrically to the phase or neutral conductor associated with said pole by virtue of suitable connection terminals.
Each electrical pole, moreover, is provided with a suitable current sensor, normally constituted either by a current sensing transformer, whose primary winding is constituted by the conductor to which said pole is connected, or by a magnetic relay whose winding coil is again constituted by said conductor.
These magnetic relays can be considered essentially as two-stage transducers, i.e., transducers capable of converting initially the electric energy into magnetic energy and then the magnetic energy into mechanical energy. The sensitive part of a magnetic relay in fact comprises an electromagnet that has a plate made of ferromagnetic material, which subjects a suitable lever system to mechanical actions that depend on the value of the magnetic field generated by the electromagnet; said magnetic field in turn depends on the intensity of the current that circulates in the corresponding electrical conductor. The mechanical actions produced in the lever system are then appropriately directed in input to a protection unit, which causes the safety release or opening of the circuit breaker.
Currently, the magnetic relays used in low-voltage circuit breakers can be of the fixed or adjustable type, the latter being the case of the present invention. In the specific case, adjustable relays allow to set the tripping threshold of the protection unit of the circuit breaker over a given range; in the current art, the methods by which the tripping threshold is adjusted allow to obtain devices that can perform their required functions adequately but have some critical aspects.
In particular, the threshold of magnetic relays is set by using suitable adjustment means, by acting on the magnetic circuit of the relay itself, which is substantially constituted by two parts: a fixed part, which comprises a ferromagnetic element, a core and a coil, and a moving part, which comprises a plate that is also made of ferromagnetic material.
There are substantially two known solutions used to adjust the tripping threshold; these solutions utilize separately two different physical phenomena: a first solution, commonly known as gap variation, entails acting on the geometric distance between the moving plate and the core; the second solution, commonly known as magnetic coupling variation, instead entails acting on the parallel sliding between the moving magnetic plate and the fixed part. In both cases, the adjustment consists in gradually modifying the relative position of the ferromagnetic plate of the moving part and the fixed part of the electromagnet, increasing or reducing the efficiency of the conversion of the magnetic energy generated by the electromagnet into mechanical energy induced in the lever system that actuates the safety release or opening device of the circuit breaker.
One of the main drawbacks of the known art arises from the fact that with these solutions the response to modifications made to the magnetic circuit is not sufficiently uniform, because identical variations in the configuration of the magnetic circuit are matched by non-uniform increases in the various stages of the adjustment range; in other words, the sensitivity of the relay tends to vary in a nonlinear fashion over the adjustment range, exhibiting a gradual degradation of reliability especially in the extreme regions of the adjustment range.
It is this last aspect in particular that leads, in common practice, to a forced limitation of the useful setting range and has a particularly negative impact especially in applications in compact circuit breakers, where limited spaces for accommodating the relays require great miniaturization, making this limitation even more significant.
Finally, since a circuit breaker is normally set by acting on multiple relays by means of a single actuation element, slight differences in movement at the level of the simultaneous adjustment devices can lead to significant imbalances in the setting among the relays of each pole and cause non-uniform tripping of the protection device.